This invention lies in the field of apparatus for transporting long loads and is directed to tractor-trailer combinations with improved steering arrangements. It is more particularly directed to such apparatus in which the steering arrangement provides protection against tipping on a fast turn and enables the tractor to negotiate turns of much smaller radius than would otherwise be possible.
Modern construction techniques frequently call for the use of very long beams of steel or reinforced concrete to serve as bridge spans or roof supports in various types of buildings. These beams may be from 50 to 90 feet in length. Reinforcing bars for on-site concrete construction may also be of comparable length. The problem of transportation of such material from a fabrication or storage location to a construction site requires special vehicles.
Probably the most generally used vehicle comprises a conventional tractor having a "fifth wheel" mount over its driving wheels and a flat bed trailer having a load bearing truck under its aft portion and a pin at its forward end for pivotal connection to the fifth wheel mount on the tractor. The load is laid directly on the bed and may overhang the aft end by several feet. The distance between the tractor driving wheels and the trailer truck is short enough to allow the total rig to negotiate reasonably sharp turns. However if the length is increased the wheel distance is also increased and maneuverability drops off rapidly so that the vehicle is not suitable for handling loads much beyond about 40 feet.
Articulated trailers having two flat beds in tandem connected on a vertical pivot axis and with trucks under each constitute an improvement but they are still unsatisfactory because the first and second trailer each travel on successively shorter radius curves than the tractor and therefore cut corners or jump curbs at typical intersections.
In another type, the aft end of the trailer bed is mounted on a truck having tandem axles spaced longitudinally a considerable distance apart with a vertical pivot connection between the bed and the truck with its axis between the forward and aft axles. A steering linkage connects the forward axle to an intermediate point of the bed and, as the bed follows the tractor around a turn, it steers the forward wheels toward the inside of the turn. The result is that both sets of wheels describe a much tighter turn and the corner cutting is even more severe.
In a variation of the type just described, the aft portion of the bed is similarly pivoted on a truck with well spaced tandem wheels, the forward wheels being non-pivoted and the rear wheels being steerable. A tongue fixedly connected to the forward end of the truck is pivotally secured to a pivot mounting at the aft end of the tractor. Lateral movement of the pivot mounting as the tractor goes into a turn tends to swing the truck toward the outside of the turn, which is desirable, but the pivotal connection to the bed requires the wheels of the forward and aft axles to swing laterally in opposite directions which would drag the wheels across the surface and generate a great deal of resistance and tire scraping. As the bed follows the tractor around a turn it swings slightly inward and steers the rear wheels toward the outside of the turn. This causes the truck wheels to follow fairly closely the path of the tractor but does not alleviate the expensive tire wear.
The two types last described are basically close coupled and have standard trailer beds, and consequently they are not suitable for truly long loads ranging from 50 to 90 feet in length. Another type which has been proposed is designed specifically for long loads. In this type, a rather short trailer bed is mounted on two longitudinally spaced trucks, each with tandem axles. It is fixedly attached to the rear truck and connected at its forward end to the forward truck by a vertical pivotal mount. A bolster is mounted on the bed at an intermediate location for rotation about a vertical axis. A long towing tongue is fixedly connected to the forward end of the forward truck and pivotally connected to the fifth wheel on the tractor. A second bolster is also pivotally mounted on the fifth wheel. The long load is laid on the two bolsters which support it at or near its two ends. When the tractor travels around a turn the forward truck follows it on a smaller radius and the aft truck follows the forward truck on a still smaller radius, with the bolsters turning on their pivots to allow the load to ride at an angle to the tractor and both trucks. Although this construction is well suited to carry long loads it does not solve the problem of corner cutting which is only aggravated by the extreme overall length of the apparatus.